This invention relates to transmission of television signals through analog channels, and to analog recording of such signals.
Analog channels for television transmission include terrestrial transmission in the VHF and UHF bands, coaxial cable, satellite transmission, and fiber-optic cable in the analog mode. Analog recording systems include tape, disk, etc. In all of these media, the received signal usually suffers loss of quality due to random noise, interference from other transmissions, multipath (echoes), and frequency distortion. This is so much the case that in broadcast television, for example, the image quality seen on home receivers is vastly reduced as compared with that produced in the studio. Reducing or eliminating these effects is especially important in high-definition TV, since channel defects might easily negate the higher resolution of which these systems are capable.
In terrestrial broadcasting, the signal-to-noise ratio (SNR) in the image is a function of the effective transmitted power, the spacing between transmitter and receiver, the effectiveness of the receiver antenna, and the quality of the circuitry in the tuner. All of these factors are limited by economics. Interference relates to the spacing between transmitters, to the antenna, and to the selectivity of the receiver circuitry. It is desired to have as many stations as possible, and the other factors are again limited by economics. Echoes due to multipath conditions can be alleviated by expensive antennas, and so are also governed by economic considerations. The channel frequency response is a function of transmitter and receiver filters, antenna characteristics, and the quality of the tuner circuitry, again economically limited.
In cable, SNR is determined by cable loss as compared with amplifier power and spacing. The pervasive microreflections result from improper terminations. The channel frequency response is affected by limitations similar to those found in broadcasting.
In principle, automatic equalizers can be used in receivers for the purpose of reducing echoes and frequency distortion. W. Ciciora et al., "Automatic Ghost Cancellation in Television Systems," IEEE Trans. on Consumer Electronics, Vol. 25, No. 1, Feb. 1979, pp. 9-54. Some noise reduction is possible using frame-recursive temporal filters, although these are rather expensive for consumer use. J. Drewery et al., U.S. Pat. No. 4,058,836. Another method of noise reduction that is quite simple is called coring. P. C. Goldmark et al., "A New Technique for Improving the Sharpness of Television Pictures," Proc. IRE, Vol. 39, No. 10, Oct. 1951, pp. 1314-1322. It is used in VCR's but produces considerable distortion. More sophisticated methods have been widely studied in the laboratory, but have not been applied to practical TV transmission because of cost and limited effectiveness. E. Dubois et al., "Noise Reduction in Image Sequences," IEEE Trans. on Communications, Vol. 32, No. 7, July 1984, pp. 502-522. D. Martinez et al., " Implicit Motion Compensated Noise Reduction of Motion Video Scenes," IEEE Proc. Intl. Conf. on Acoustics, Speech, and Signal Processing, Mar. 26, 1986, pp. 375-378. Some noise-reduction methods are employed in tape recorders. See U.S. Pat. Nos. 4,607,285; 4,618,393; and 4,007,483.
An additional factor not always considered in rating television systems is interference performance--both the degree of interference caused in other signals and the ability of the signal to deal with interference received from other signals. Improvements in interference response would permit closer transmitter spacing. "Spectrum Compatible HDTV System," Zenith Electronics Corp., Glenview, Ill., Sept. 1, 1988.
My pending application, hereby incorporated by reference, U.S. Ser. No. 149,673, filed Jan. 28, 1988, entitled "Improved Definition Television Systems," describes a technique known as subband coding. In this class of methods, the 3-dimensional spectrum of the video signal produced by a high-quality camera is divided into spatiotemporal components. Each component is selected by a filter, preferable separable. That means that a cascade of three 1-dimensional filters--vertical, horizontal, and temporal--is employed. The components are time-multiplexed for transmission, each component being independently processed, for example by a static nonlinear amplifier and by an adaptive modulator. W. F. Schreiber et al., "Channel-Compatible 5-MHz HDTV Distribution System," Presented at SMPTE Conference, Nashville, Jan. 29, 1988. At the receiver, each component is demodulated and then all the components are added up to produce the reconstructed video signal. The advantage of this technique is that the perceptually more important components can be used for transmission, adaptively selected, if desired, and each component can be treated appropriately according to how it is perceived.
My copending application, also incorporated by reference, U.S. Ser. No. 61,140, filed on June 10, 1987, entitled "Improved Transmission of Signals Through Analog Channels Using Adaptive Frequency Modulation", describes improvements in quality obtainable with adaptive modulation. It is the nature of television images that they consist of objects of varying sizes, mostly much larger than a single picture element (pel). Thus, most of the pels in an image are located in the interior of objects and many fewer at the edges, where the video signal changes substantially from pel to pel. The human visual system is much more sensitive to noise in the relatively blank areas within objects than in the busy areas at the edges of objects or where there is a great deal of fine detail. It is just in these relatively blank areas where all the signal components (except the one containing the dc level and the low spatiotemporal frequencies) are quite small. These small signals can be greatly increased for transmission and correspondingly reduced (along with the noise) at the receiver. Adaptive modulation thus greatly reduces the noise of whatever origin in just those areas where it would be most visible. The efficacy of adaptive modulation is highest for those signals that are very small in the blank areas--namely the high-frequency signals. The dc and low-frequency RGB signals (or the luminance/chrominance signals, in an L/C system) do not have this property.
It is well known that noise of all kinds can be essentially eliminated in most cases by digital transmission. This is used in some fiber-optic transmission lines and in certain disk-recording systems. It is not used for normal TV transmission because it requires a very large increase in channel bandwidth.
Frequency modulation is another method of improving SNR by using a wider bandwidth. It is used in satellite transmission, where wide bandwidths are available. A narrow-band form of FM is also used in magnetic video recording to reduce the effect of unwanted changes in signal level. It is not used in normal TV transmission because of the wide bandwidth required.